What does 'Mark Sharp' *actually* do?
According to the v2.92 manual (emphasis added):

The Sharp mark is used by the split normals and the Edge Split modifier, which are part of the smoothing or customized shading techniques.

So, anything that uses split normals can also use the sharp data.  This applies to the Auto Smooth feature of meshes.
Per the manual:

By default in Blender . . . a sharp edge is always defined as an edge being either non-manifold, or having at least one of its faces defined as flat.
Enabling the Auto Smooth setting adds an extra parameter to define a sharp edge, the Angle threshold between two neighbor faces, above which the edge will always be considered as sharp.

So, for example, you could apply Auto Smooth, say at 30°.  Then you could mark an edge as sharp, say an edge that was only 20°.  That edge would stay sharp thus overriding the Auto Smooth.